60 PRINCIPLES OF PALAONTOLOGY. 
those inhabiting our present oceans, we should in most in- 
stances find differences so great as almost to place us in 
another world. This divergence is the most marked in the 
Paleeozoic forms of life, less so in those of the Mesozoic period, 
and less still in the Tertiary period. Each successive formation 
has therefore presented us with animals becoming gradually 
more and more like those now in existence ; and though there 
is an immense and striking difference between the Silurian 
animals and those of to-day, this difference is greatly reduced 
if we compare the Silurian fauna with the Devonian; shat 
again with the Carboniferous; and so on till we reach the 
present. 
It follows from the above that the animals of any given 
formation are more like those of the next formation below, 
and of the next formation above, than they are to any others ; 
and this fact of itself is an almost inexplicable one, unless we 
believe that the animals of any given formation are, in part at 
any rate, the lineal descendants of the animals of the preced- 
ing formation, and the progenitors, also in part at least, of the 
animals of the succeeding formation. In fact, the paleon- 
tologist is so commonly confronted with the phenomenon of 
closely-allied forms of animal life succeeding one another in 
point of time, that he is compelled to believe that such forms 
have been developed from some common ancestral type by 
some process of “evolution.” On the other hand, there are 
many phenomena, such as the apparently sudden introduction 
of new forms throughout all past time, and the common occur- 
rence of wholly isolated types, which cannot be explained in 
this way. Whilst it seems certain, therefore, that many of the 
phenomena of the succession of animal life in past periods can 
only be explained by some law of evolution, it seems at the 
same time certain that there has always been some other 
deeper and higher law at work, on the nature of which it 
would be futile to speculate at present. 
Not only do we find that the animals of each successive 
formation become gradually more and more like those now 
existing upon the globe, as we pass from the older rocks into 
the newer, but we also find that there has been a gradual pro- 
gression and development in the Zyfes of animal life which 
characterise the geological ages. If we take the earliest-known 
and oldest examples of any given group of animals, it can 
sometimes be shown that these primitive forms, though in 
themselves highly organised, possessed certain characters such 
as are now only seen in the young of their existing representa- 
tives. In technical language, the early forms of life in some 
